Prepreg sheets are used in the manufacture of printed circuit boards. A prepreg sheet is a glass fabric which has been impregnated or coated with an epoxy resin and the epoxy resin advanced to the B-stage which is 30-40% of complete cure. The prepreg sheets are laminated together by heat and pressure to form the board. The inner prepreg sheets of the laminate are used for bonding purposes and to make up the desired dielectric thickness of the board. Copper plating forming the printed circuitry is applied to one or both of the outside prepreg sheets of the board.
One preferred form of applying the copper is by the well known electroless deposition process. However, prior to depositing the copper, the surface of the cured epoxy resin must be conditioned by roughening or the like so that the deposited copper pattern will adequately adhere to the resinous surface. There are several ways of conditioning the epoxy surface and one preferred method, which is fully described in U.S. Pat. No. 3,808,028, includes first exposing the cured epoxy surface to a suitable swellant of the cured epoxy. The cured surface is exposed to the swellant for a period of time sufficient to swell the epoxy surface. The swelled epoxy surface is then exposed to an etchant for the cured epoxy for a sufficient period of time to etch the swelled surface and put the surface in a condition of improved adherence for metal deposits, and the like.
In spite of the improved conditioned epoxy resin surface produced by the chemical swell and etch method, adequate adhesion of the copper plating to the resin surface still presented a problem in the manufacture of laminates for printed circuit boards. Heretofore, the standard practice of making prepreg sheets involved advancing the epoxy resin to the normal B-stage which is 30-40% of a complete cure. Upon laminating the sheets together to make a board using, for example, 500-900 p.s.i. at 350.degree. F, the epoxy resin was unable to withstand the required lamination temperature and pressure that is needed for epoxy glass resulting in the epoxy resin flowing and squeezing out so that an insufficient amount of resin was left on the surface. When the subsequent chemical swell and etch operation was carried out, the etching would expose the glass fibers of the prepreg sheet which resulted in the formation of blisters and reduced copper adhesion. To alleviate this condition, a resin dip coating operation was introduced after the prepreg sheets were laminated together and before the swell and etch process was carried out. This step proved to be cumbersome and costly. Also, dip coating after laminating left defects, such as, unevenness and pin holes. The bottom of the coating would be heavier than the top and also de-wet areas, such as, cratering and oil spots would form. There was also tried out, an initial pass before lamination of a heavier coating of resin but this did not work out because not a sufficient amount of resin could be obtained which would level out and maintain the desired 0.001-0.002 inch thickness that was desired above the glass fibers.
Another proposed solution to the problem is described in U.S. Pat. No. 3,698,940 and involves painting a first coat of resin varnish on the glass cloth and curing it to the B-stage followed by painting a second coating of resin varnish over the first coat and curing this coat to the B-stage. This is followed by curing using a dwell lamination cycle at low pressure, the purpose of which is to flow the resin and advance it beyond the B-stage to insure that the resin will stay, and this is followed by another lamination cycle at high pressure. In this type of process, the prepreg coatings had wide tolerances which necessitated varying dwell lamination cycles. As a result, it is very difficult to control the process and maintain the resin where you want it to obtain the desired uniformity of coating. It became apparent that some other means would have to be devised to obtain the desired resin rich surface for the outer prepreg sheets of a printed circuit board.